A ferroelectric liquid crystal is capable of exhibiting a plurality of optical states, and can continue to retain a particular state even after removal of applied voltage. When an external force, such as an electric field is applied, ferroelectric liquid crystal molecules lie in one of two stable positions on the lateral surface of a cone (liquid crystal cone). In a liquid crystal display device constructed by sandwiching such a ferroelectric liquid crystal between a pair of substrates, the ferroelectric liquid crystal is controlled so that the liquid crystal molecules lie in one of the two stable positions in accordance with the polarity of the voltage applied across the ferroelectric liquid crystal. One of the two stable states is the first ferroelectric state, and the other is the second ferroelectric state.
FIG. 1 is a diagram illustrating an exemplary configuration of a ferroelectric liquid crystal panel 20 that uses a ferroelectric liquid crystal 10.
In FIG. 1, polarizers 15a (the direction of its polarization axis is designated by “a”) and 15b (the direction of its polarization axis is designated by “b”) are arranged in a crossed Nichol prism configuration. Ferroelectric liquid crystal 10 is oriented so that the long axis direction of the liquid crystal molecules in ferroelectric liquid crystal 10 in the second ferroelectric state coincides with the polarization axis “a”. Accordingly, in the first ferroelectric state, the long axis direction of the liquid crystal molecules coincides with the direction of the other position on the liquid crystal cone as illustrated in FIG. 1.
When polarizers 15a and 15b and ferroelectric liquid crystal 10 are arranged as illustrated in FIG. 1, and ferroelectric liquid crystal 10 is placed in the second ferroelectric state by changing the polarity of the applied voltage (the long axis direction of the liquid crystal molecules in ferroelectric liquid crystal 10 coincides with the polarization axis “a” of polarizer 15a), light is not transmitted through the liquid crystal, and thus ferroelectric liquid crystal panel 20 produces a black display (non-transmission state). On the other hand, when ferroelectric liquid crystal 10 is placed in the first ferroelectric state by changing the polarity of the applied voltage (the long axis direction of the liquid crystal molecules in ferroelectric liquid crystal 10 do not coincide with either the polarization axis “a” of polarizer 15a or with the polarization axis “b” of polarizer 15b), since the long axis direction of the liquid crystal molecules is tilted at a certain angle relative to the polarization axes, light, for example, from a backlight is transmitted through the liquid crystal and liquid crystal panel 20 thus produces a white display (transmission state). A light source other than the backlight may be used to produce the display.
FIG. 2 is a diagram illustrating the relationship between applied voltage and light transmittance for the ferroelectric liquid crystal.
As illustrated in FIG. 2, when the voltage applied to ferroelectric liquid crystal 10 is increased, the voltage value at which light transmittance begins to increase is denoted by V1, and the voltage value at which the transmittance reaches saturation when the voltage is further increased is denoted by V2 (positive threshold). Then, when the voltage applied to ferroelectric liquid crystal 10 is decreased, the voltage value at which the transmittance begins to drop is denoted by V3, and the voltage value at and beyond which the transmittance does not drop further even when the voltage is further decreased is denoted by V4 (negative threshold). The state in which the transmittance is high corresponds to the first ferroelectric state, and the state in which the transmittance is low corresponds to the second ferroelectric state.
For example, when a voltage larger than V2 is applied to ferroelectric liquid crystal 10, ferroelectric liquid crystal 10 is switched to the first ferroelectric state, and thereafter ferroelectric liquid crystal 10 retains the first ferroelectric state even when no voltage is applied. Likewise, when a voltage larger in magnitude than V4 is applied to ferroelectric liquid crystal 10, ferroelectric liquid crystal 10 is switched to the second ferroelectric state, and thereafter ferroelectric liquid crystal 10 retains the second ferroelectric state even when no voltage is applied. In this way, once switched to a given ferroelectric state, ferroelectric liquid crystal 10 retains that state even after removal of the applied voltage. Such a ferroelectric liquid crystal is described, for example, in Patent Document 1.
Further, in the liquid crystal display device using ferroelectric liquid crystal 10, in view of the fact that the thresholds for switching between white and black displays highly depend on the condition of the pixels immediately before the write operation, there is known a liquid crystal driving method that, when the voltages are applied to the pixels, a first voltage V1 is applied and, then, a second voltage V2 (V1<V2) is applied so that the pixels can be switched regardless of the immediately preceding condition of the pixels. (For example, see Patent Document 2.) However, Patent Document 2 does not describe the problem that a clear image cannot be obtained due to threshold unevenness of ferroelectric liquid crystal 10 and provides no solution.
Further, in the liquid crystal display device using ferroelectric liquid crystal 10, it is known that four reset pulses (a neutralization part of a black display level reset pulse, a black level reset pulse, a neutralization part of a white display level reset pulse, and a white level reset pulse) are used to uniformly restore the director of the liquid crystal changed by the previously applied voltage. (For example, see Patent Document 3.) However, Patent Document 3 does not describe the problem that a clear image cannot be obtained due to threshold unevenness of ferroelectric liquid crystal 10 and provides no solution.    Patent Document 1: Japanese Unexamined Patent Publication No. 2006-23481 (FIGS. 1 and 2)    Patent Document 2: Japanese Unexamined Patent Publication No. H4-85517 (FIG. 1)    Patent Document 3: Japanese Unexamined Patent Publication No. H9-54307 (Page 9, FIG. 1)